Molecular Simulation Study Of Fluid Adsorption And Diffusion In Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs), commonly recognized as "soft" analogues of zeolites, is a new class of nanoporous materials. MOFs, with extremely high porosity, chemical diversity, and as tailored materials with well-defined pore size, are promising materials for gases storage, separation, and catalyst, etc. Computational chemistry can not only overcome the limitations of traditional methods, but also provides theoretical guidance for the design of optimal adsorbents and the determination of optimal industrial operation conditions, which also saves a lot of time for complicated experimental works. In this work, a systematic study was carried out on gas storage, separation and diffusion in MOFs using molecular simulation technique. The main contents and findings are summarized as follows.(1) Firstly, a systematic molecular simulation study was performed to investigate the effect of catenation on methane adsorption in MOFs. It shows that catenation can greatly enhance the storage capacity of methane in MOFs, and meet the DOE target for methane storage in porous materials easily. (Chapter 2)Secondly, by the modification of an catenated MOF named IRMOF-15, two new IRMOFs were designed that show largely improved methane storage capacity, which exceed the DOE target and give comparable values to the available MOF (PCN-14) with highest methane storage capacity on volume basis, but much higher values on weight basis. (Chapter 2)Finally, molecular dynamics simulations were performed to investigate the effect of catenation on methane diffusion as well as diffusion mechanism in those materials. The results show that the diffusivity was reduced ca. 3～5 times by the presence of catenation, which is much larger than that on hydrogen at room temperature. A detailed analysis of diffusion mechanism reveals that the diffusion pathways of methane molecules in catenated MOFs are mainly governed by the strong confinement in catenation regions, leading to a 3D diffusion along the pathway from one catenated region to another. The results obtained provide useful information that can guide the future design of MOFs for methane storage and separation applications. (Chapter 3)(2) The modified MM3 force field for describing flexible IRMOF-1 was extended to include other IRMOFs, and a molecular dynamics simulation study was performed on hexane diffusion in IRMOF-1 and IRMOF-16. The self-diffusion coefficients and diffusion pathways of hexane, as well as the mobility of the frameworks were investigated, as a function of both temperature and loading. The results revealed that the diffusion pathway of hexane was largely influenced by loading, and the flexibility of IRMOF-16 is much larger than that of IRMOF-1. The microscopic information obtained is useful for understanding the diffusion mechanism of chain molecules in dynamic MOFs. (Chapter 4)(3) The effects of the side pockets on gas separation in Cu-BTC were studied. The results show that the side pockets not only affect the selectivity behavior largely, but also enhance selectivity significantly. At low-pressure regime, the selectivity can be enhanced approximately by 2-3 times, while at moderate pressures for practical application, the enhancement is also evident (ca. 30%-50%). This work also shows that the contribution of the side pockets can be related to the interactions between adsorbate and Cu-BTC: the larger the difference in interactions of adsorbates with Cu-BTC, the bigger the contribution of the pockets. (Chapter 5)...